1. Field of the Invention
The present invention relates to an image heating apparatus for heating an image formed on a recording material. Examples of the image heating apparatus include, for example, a fixing apparatus for fixing an unfixed image on the recording material, and a gloss improving apparatus for improving gloss of an image by heating an image fixed on the recording material. The image heating apparatus is used in an image forming apparatus such as a copying machine, a printer, a facsimile, and a composite machine having a plurality of functions thereof.
2. Description of the Related Art
Recently, regarding a fixing apparatus, energy-conservation is becoming active, and the reduction in a rising time is considered.
As one means, a belt fixing system has been proposed, in which a belt-shaped endless belt (hereinafter, referred to as a “fixing belt”) is used as a heating rotary member, and toner on a recording material is heated via the belt heated by a heater.
A belt fixing apparatus is proposed, for example, by Japanese Patent Application Laid-open No S63-313182, Japanese Patent Application Laid-open No. H02-157878, Japanese Patent Application Laid-open No. H04-44075, and Japanese Patent Application Laid-open No. H04-204980.
In the belt fixing apparatus, a fixing belt is sandwiched between a ceramic heater serving as a heating member and a pressure roller serving as a pressure member, whereby a fixing area (i.e., fixing nip portion) is formed. A recording material on which an unfixed toner image is formed and carried is introduced between the fixing belt and the pressure roller in the fixing area, and the recording material is transported under the condition of being sandwiched therebetween together with the fixing belt. Consequently, the unfixed toner image is fixed onto the surface of the recording material with a pressure force of the fixing nip portion while the heat of a ceramic heater is given via the fixing belt.
Such a fixing apparatus uses a member with a low heat capacity for the fixing belt. Therefore, there is an advantage that the waiting time from the power-up of an image forming apparatus to a state where an image can be formed is short (i.e., quick start property), the power consumption during stand-by is remarkably small (i.e., low power consumption), etc.
In order to fix a recording material with a largest length in the width direction (hereinafter, referred to as a maximum-size recording material), for example, the entire area of an A4 landscape sheet (size: 297 mm), it is preferable to heat a portion in the width direction of the fixing belt to a temperature equal to or higher than that of a portion in the width direction of the maximum-size recording material. However, a recording material with a length in the width direction smaller than that of the maximum-size recording material (hereinafter, referred to as a small-size recording material), for example, an A4 portrait sheet (size: 210 mm) is continuously supplied, the temperature in a non-sheet passing area of the fixing belt rises excessively. Therefore, when a maximum-size recording material is supplied after the continuous sheet supply of a small-size recording material, hot-offset occurs in a portion of the small-size recording material corresponding to the non-sheet passing portion, which remarkably degrades image quality. Alternatively, when a small-size recording material (e.g., a B4 portrait sheet) with a length in the width direction larger than that of the small-size recording material (e.g., an A4 portrait sheet) that is continuously supplied, hot-offset occurs in a portion of the former recording material corresponding to the non-sheet passing portion, which remarkably degrades image quality.
In order to prevent a hot-offset phenomenon occurring along with the excessive increase in temperature of the non-sheet passing area of the fixing belt, in a conventional fixing apparatus, self-radiation cooling is allowed to be performed until the temperature of the non-sheet passing area of the fixing belt decreases sufficiently after the continuous sheet supply of small-size recording materials. Then, after the temperature distribution over the entire area in the width direction of the fixing belt becomes substantially uniform, maximum-size sheets or the like are supplied continuously.
However, in order to make the temperature distribution over the entire area in the width direction substantially uniform by self-radiation cooling, a cooling time of about several seconds to several minutes (hereinafter, referred to as a “downtime”) is required. That is, the subsequent sheet cannot be supplied by the downtime, which prevents the enhancement of productivity.